Sintered metal article and method of making same



United States atent 3,142,894 SINTERED RETAL ARTKCLE AND METHGD FlVlAKlNG SAME Stuart T. Ross, Bloomfield Township, Oakland County, andWalter E. .iominy and Frederick A. Hagen, Detroit, Micln, assiguors toChrysler Corporation, Highland Park, Mich, a corporation of Delaware NoDrawing. Continuation of application Ser. .No. 774,166, Nov. 17, 1958.This application Aug. 31, 1962, Ser. No. 220,854

20 Claims. (Cl. 29182.5)

This invention relates to sintered powdered metal articles andstructures adapted for operation in relatively high temperatureapplications, as high as or even higher than 1200 F.

It particularly relates to sintered articles and structures of thiskind, for instance seal and bearing structures characterized bysubstantial dimensional and physical stability at such high temperaturesfor substantial periods of time and operative at these temperatures andat lower temperatures, including ambient temperatures under oxidizing,reducing and neutral atmospheric conditions without externallubrication. Moreover, our invention is concerned with the novelcompositions and methods for obtaining these features.

The present application is a continuation of co-pending applicationSerial No. 774,166 now abandoned filed November 17, 1958.

It has heretofore been proposed as, for example, in the patents toCalkins 1,974,173 and 1,940,294 to make sintered powdered products ofcompositions essentially of iron particles, powdered copper, and smallamounts less than 2% of powdered graphite. These compositions aftermixing are conventionally compacted in molds at pressures of between25,000 to 50,000 p.s.i. and then sintered at about 2100 F. in a reducingor non-oxidizing atmosphere and then cooled in such atmosphere.Customarily it has also been the practice to incorporate quantities oflubricant such as oil or soap in the composition after sintering. Thesecompositions produce porous structures containing substantial amounts ofcoarse pearlite and considerable ferrite (Fe) having use as internallylubricated bearing materials, at low temperatures. However, they lackthe wearing properties and the physical and dimensional stabilityessential for use as hearing materials or seals at temperatures in theorder of 900 F. to 1200 F. Furthermore, such materials are not capableof providing good wearing properties without lubrication under anyconditions.

It has also been proposed in the British patent No. r

719,146 to Deventer to consolidate a cold mixture of very fine particlesof iron, copper, and between 2 to graphite at a pressure of 1000atmospheres (about 15,000 p.s.i.), and to then subject the compact to apressure of 3500 atmospheres (52,500 p.s.i.) in a furnace heated at 960C. (1760 F.) and to then cool the material to a temperature of 150 to300 C. while being maintained under this high pressure. It isspecifically stated that the material is fritted and not sintered, whilethe pressure is maintained, until the mixture becomes coherent. Thepatentee suggests that the product provides a bearing surface whichneeds no lubrication at elevated temperatures (not stated) but which,from the uses suggested by the patentee, to Wit, piston rings, bearings,crosshead checks are applications not involving temperatures much above400 F. Due to the low temperature of heat treatment in the Deventerprocess, which for all purposes may be denominated as a hot pressingprocess, during which no liquid phase sintering or carburizing occurs,the resulting product is essentially composed of ferrite and graphiteand ice is substantially free of pearlite, hematite, magnetite, wiistiteand carbides, one or more of which it will be seen are essential to thepresent invention. Moreover, the Deventer product is substantiallyweaker than those of Calkins and hence can be used only under conditionsof light stress. It also obtains no advantage whatever from a liquidphase copper and when used at elevated temperatures as high as 1200 F.the pores of this material become rapidly filled with oxidation productsproducing swelling and dimensional changes. Moreover, its tensilestrength is then only about 1600 p.s.i.

It is further proposed in the patent to Lenel 2,187,589 to treat withsteam, air, carbon dioxide or oxygen at 1,050 F. a porous, sintered ironarticle fabricated from iron or iron compositions including graphite,copper, nickel, or manganese and in particular a mixture of 98 partssponge iron, 2 parts graphite and 1 /2 parts Zinc stearate or stearicacid, to obtain a corrosion-resistant film or layer of oxide which willprevent corrosion of the product at normal temperatures by the action ofthe atmosphere or water vapor. t is stated that a film of magnetic ironoxide will form a better corrosion-resistant and adherent film than rediron oxide.

The patentee is concerned with the corrosion aspects of porous ironproducts at ambient temperatures. There is no recognition of theadvantages to be obtained by iron oxide formation in a sintered productin conjunction with adequate amounts of graphite from the standpoint ofproviding a wearing surface for seals or bearings to be subjected tooperation at high temperatures in the order of 1200 F. and higher. Noris it recognized that oxide formation prior to use facilitates themaking of a product having substantial dimensional stability insubsequent high temperature use.

There are many applications where a material is required to serve, forexample, as either a seal or bearing at high temperatures above 900 F.,in the order of 1200 F. to 1400 F. and where the parts at thesetemperatures must have substantial strength, must be dimensionallystable and/or must resist wear by rubbing. Moreover,

due to the high temperatures of operation, and in many instances theconfiguration of the parts involved, lubrication is not possible. A casein point is the regenerator of a gas turbine engine. In such aninstallation it is essential that the hot exhaust gases and ambienttemperature incoming air be sealed from each other as they enter andleave the regenerator. The temperature of the oxidizing gases involvedin such an operation covers a range of between 1200 F. to 1400" F.Moreover, the regenerator surface is not flat. Hence, the seal must beflexible enough to conform under spring loading, to its particularcontour or any changes therein during operation. Since lubrication isnot possible, the seal must be made of a material that will operatesatisfactorily for many hours (preferably above 500) under the statedconditions. A further example is the regenerator center bearing.- Thispart is required to function as a bearing under oxidizing conditions atthe indicated temperatures without lubrication while at the same timeresisting wear by rubbing and being substantially dimensionally stable.Similar requirements are essential in the case of missile parts except,however, that there, dimensional stability is not of greatest importancebecause of the limited service expected.

The known products discussed above will not provide the desired featuresand parts made from unprotected graphite alone have no strength whateverat the stated temperature under extended exposure due to catastrophicoxidation.

It is therefore the main object of our invention to provide a powderedmetal article or product comprising a sintered compact of powderedferrous metal and graphite with or without copper in which the ferrousmetal is preferably substantially pure iron powder but may be a powderedalloy of iron, and which product will exhibit substantial strength, ahigh degree of dimensional stability and resistance to substantialstressing when exposed to heat, especially temperatures above about 900F. and as high as about 1200 F. and even higher, for substantial periodsof time and that will, depending upon treatment, have an oxidized grainstructure characterized by the appearance of substantial amounts of oneor more surface oxides from the group consisting of magnetite, w'Listiteand hematite between and around particles of ferrite and partiallydecarburized and/ or spheroidized pearlite.

Another object is to provide a powdered metal article or product as inthe preceding object, such as a seal or bearing and the like comprisinga sintered compact of powdered ferrous metal with or without copper and/or alloying metals, that includes graphite in amount sufficient tominimize wear during the breaking-in period byserving as a lubricantduring such period and that will promote Wear resistance and control thewear rate at the mating surface during service both by dry filmlubricant action and by its modifying effect on the surface oxides toform those providing a compensating growth.

A further object is to provide an article or product as in the precedingobjects which requires no external lubrication even at hightemperatures.

It is also an object to provide a novel process by which products of thecharacter recited in the preceding objects may be obtained.

A particular object is to improve the dimensional stability of sinterediron powdered metal articles and products in high temperature service bysubjecting the products following treatment at the sintering temperatureto cooling in a reducing atmosphere to a temperature below that whichwill produce catastrophic oxidation which temperature may be ambienttemperature, and preferably during this cooling cycle which may alsoinclude holding at such a temperature or, following reheating in airafter completing this cooling cycle, in either event, from a temperaturebetween 1050 F. to 2150 F.', cooling the" product in an oxidizingatmosphere such as air, steam or oxygen to a temperature at whichnonuniform cooling thereafter outside the furnace will not promotewarpage from thermally induced stresses.

These and other objects and advantages of our invention will be apparentfrom the following description.

We have now discovered and believe that the dimensional instability ofprior products referred to above is primarily due to a growth in theparticle structure occasioned by the after formation especially at hightemperatures above 900 F., of substantial amounts of the iron oxide (FeO known as magnetite and/ or the iron oxide (FeO) known as wiistiteduring oxidation at such high temperatures. This condition appears tofollow decarburization of the pearlite and cementite produced insintering and the subsequent oxidation to magnetite of the ferrite thusformed or present. This condition can be substantially avoided and asubstantially dimensionally stable product be obtained by providing inthe sintered matrix prior to subjection of the products to hightemperature service an oxidized grain structure providing substantialamounts of desirable oxides from the group of magnetite, Wiistite andhematite over, between and around particles of ferrite and partiallydecarburized and/or speroidized pearlite so as to produce a surfacelayer or layers thereof. Moreover, through the provision of sufiicientgraphite tocontrol break-in wear and to control further oxidation wehave found that this substantially stable condition once obtained can besubstantially preserved in service.

Further, We have found that the poor wearing properties evident in theprior structures are due to their inherent composition of pearlite and/or ferrite which permits wear of the soft iron by smearing and galling.This condition can be inhibited by the presence of substantial amountsof oxide at the wearing surface of the article, a feature of ourinvention which is found to act as a wear-controlling agent. It reducesthe wear rate in service at the mating surfaces and provides a structurerequiring no external lubrication. The preferred oxide is usuallyidentifiable at the wear interface as a smooth black shining non'abrasive surface.

We have discovered that the foregoing advantageous products and novelgrain structure is possible of attainment prior to service use by anumber of procedures of which a few hereafter described will be by wayof example, all, however, involving oxidizing of the sintered matrixunder prescribed controlled conditions.

Thus the compacted ferrous metal composition will be sintered underreducing conditions, generally at atmospheric pressure at a temperaturebetween 1900 F. and

2300 F., high enough to obtain a bond between the ferrous particles andrender any copper present, in the fluid phase. The temperature employedwill depend upon the nature of the ferrous particles making up thecompact. For example, where the ferrous particles are of substantiallypure iron sintering will preferably be carried on at a temperaturebetween 2000 F. and 2050 F. for best results.

The sintering treatment produces a relatively dense ferrous structure ormatrix of integrally bonded ferrous particles whose grain structure ischaracterized by large particle areas of pearlite and ferrite orcementite. When present, the copper aids to improve the bond between theferrous particles.

The hot sintered matrix is now cooled in a reducing atmosphere such asdry hydrogen or cracked gas to a temperature at which the atmosphere maybe changed to an oxidizing atmosphere such as air, steam, or oxygen,which temperature will be one dependent upon the character of ferrousparticles in the composition, the density of the matrix, the time atsuch temperature, and the rate of subsequent cooling.

In all cases the combination of temperature, time and rate of coolingwill be employed to avoid catastrophic oxidation. For most operationsthe temperature will fall in the range 1050" to 2150" F. with a coolingrate where a thermal oxidation treatment is employed of between 50 F. to500 F. per hour, the higher the temperature the higher the cooling rate.Also where isothermal oxidation treatment is employed the rate ofcooling will preferably be greater. Where the ferrous particles are, forexample, of substantially pure iron a good temperature will be about1500 F. with a cooling rate of at least about F. per hour.

The still hot matrix is now substantially uniformly further cooled inthe oxidizing atmosphere from this temperature at the controlled ratedown to a temperature where further cooling of a non-uniform character,such as cooling outside a furnace, will not produce warpage due tothermal stressing. This temperature will generally be between about 300F. to 500 F. Uniform cooling to ambient temperature is preferred.

I11 many cases it may be preferred for manufacturing reasons and ease ofhandling the parts to initially cool the hot sintered matrix in thereducing atmosphere to a temperature below that where the change from areducing to an oxidizing atmosphere would take place as described aboveand which temperature may be as low as ambient temperature. In suchevent the matrix will be reheated in air or a reducing atmosphere to theaforesaid atmosphere change-over temperature and then substantiallyuniformly cooled as described above in an oxidizingatmosphere as whereno reheating was involved. In some cases rapid reheating may beessential, in which case induction heating is preferred. The recoolingrate will be determined by the factors previously described to which maybe added the reheating time and atmosphere in which reheating takesplace.

The described cooling of the matrix in an oxidizing atmosphere willreduce the amount of pearlite by decarburization and promote substantialinternal oxidation throughout 30 to 60% of the cross-sectional area ofthe part to produce a particle growth of iron oxide as represented byone or more of magnetite (Fe O wiistite (FeO), and hematite (Fe O Theresulting product will be stable to changes in dimension when subjectedto heating in service, especially to high temperatures above about 900F. and up to about 1200 F. and even higher and will exhibit excellentwearing properties in such use. The iron oxide will generally constitute5% to 20% by volume of the internally oxidized areas. For optimumstrength and dimensional stability this volume should preferably be atleast about Where optimum stability is not of greatest importance thispercentage may be as low as 2% We have further discovered that whengraphite is incorporated in the composition in the proper amount eitherlocally or throughout the same so as to be present in the as-sinteredwearing surface, it serves to minimize initial wear by acting as alubricant during the breaking-in period. Moreover, where the sinteredproduct has also been given the dimensional stabilizing oxidizingtreatment of the invention, such graphite both at the wearing surfaceand throughout the structure inhibits a modification of the desiredoxide present as a result of such treatment. Furthermore, there isevidence that it promotes the formation of additional iron oxide,(magnetite) and/ or wiistite (FeO). The additional magnetite and/ orwiistite formed during service further reduces the wear rate at themating surfaces by providing a compensating growth without incurringdimensional instability. Furthermore, the graphite at the wearingsurfaces minimizes galling wear by acting as a dry-film lubricant in lowtemperature service below about 900 F.

Although the dimensional stability features of our invention and theprocessing making this possible have broad application to products ofsintered powdered iron per se, it is preferred for reasons alreadydescribed and hereafter expanded that the composition of the products ofour invention consist essentially of iron, copper and graphite. Theamounts of each to be used will depend somewhat upon the size andcomplexity of the part to be made, the green strength of the compact,and the service the finished product is to encounter. In general, inorder to obtain the maximum novel benefits of the invention, theingredients of such preferred composition are preferably keptsubstantially within the following range:

Percent by weight Copper 1 /2 to 6 Graphite 6 to Iron Balance The upperlimit of graphite is limited by the green strength required of thecompact. Amounts should not be used in excess of that required toproduce a compact that will stay together during the in-process handlingand sintering, generally without use of extraneous binders, using only awetting agent, such as kerosene, which will distill out long beforesintering occurs. This amount of graphite is usually approximately 15%by weight.

In certain cases as where large or complex shapes, such as turbineseals, are to be made it is found that the sprin out after elasticcompression of the graphite in the briquetting die may cause a change ingreen shape of the piece or even prevent its ejection from the die. Inthese cases an upper limit of 12% by weight of graphite is preferred.

We have further found that the use of about 10% graphite will give aboutspring-out in forming and about V2 shrinkage on sintering, creating anapproximate balance with resultant exactness in the size of the product.By using this amount of graphite it is possible to make the diessubstantially size-for-size, according to the dimensions of the finishedproduct as shown by print. This is a distinct advantage from aproduction standpoint.

As previously described, it is desirable to have sufiicient graphitepresent such that additional magnetite is formed during service at hightemperatures. Without any graphite present, the pre-oxidizing treatmentof the invention will produce sufiicient iron oxide to preventsubstantial dimensional instability, the control of which is animportant feature of our invention, but such will not preventmodification of the oxide in service at high temperature with consequentloss of wear resistance. The presence of adequate graphite is needed topromote the formation of additional magnetite desirable for wearcontrol.

We have found that at least about 6% graphite is required in thecomposition to provide the desired control at the wearing surface underoxidizing conditions at temperatures in the range 900 F. to 1200 F. Atlower temperatures the graphite oxidizes to CO and CO at a much slowerrate, therefore a greater quantity of graphite is needed for suchservice.

In general, we have found that the use of about 10% graphite will givethe best combination of green strength, sintered strength, and hightemperature, and break-in period wear properties.

Examples of commercial graphite powders we can use are Dixon 8485graphite, a natural graphite made by Joseph Dixon Crucible Company,Jersey City, New Jersey, and Asbury F. G. graphite, a natural graphitemade by Asbury Graphite Mills, Asbury, New Jersey, both of which containabout carbon and 5% inert material. By preference, the graphite will bein fine powdered form, preferably of a size to pass through a 325 meshscreen. When of this size it will readily distribute uniformly in thecomposition when wetted by kerosene and blended with iron powders of 200to 325 mesh. Larger size graphite may be used but such will affectuniformity and tend to produce soft areas in the wearing surface whichwill be swept out in the wearing process. The Dixon graphite is of suchfineness that 97% will pass through a 325 mesh screen and the Asburygraphite is even finer being all minus 325 mesh in size.

In the powdered iron compositions of our invention the copper serves todensify the iron by liquid phase sintering and aid in the bond betweeniron particles and to thus impart strength. About 4% of this ingredientis optimum but we can use up to about 6% and as little as 1 /2% to 2%with good results. As previously stated, the copper may be entirelyomitted but as in the case of omitting graphite, such has undesirablefeatures. When the copper is omitted, it is essential to sinter attemperatures higher than 2050 F. or for longer periods than one-halfhour to attain equivalent densification. Moreover, in such casessubstantial amounts of undesired primary carbides may form.

Examples of finely divided powdered copper we can use, are electrolytictype powdered copper such as electrolytic type C copper and electrolytictype ML copper made by American Metal Climax, Inc., New York, New York.The copper will preferably be between 100 to 325 mesh size. Copper C is95% minus 325 mesh, and ML copper is all minus 100 mesh and 70% minus325 mesh. The size of the copper particles is not of great importance asit melts in sintering. However, the finer sizes are preferred because itaids in briquetting.

A portion or all of the copper may, when desired, be replaced by a metalsuch as tin or lead or combinations thereof having a melting point belowthe sintering temperature used and which substituents are not strongdeoxidizers (i.e., like aluminum) and will have boiling points above thesintering temperature. Moreover, alloys of copper, tin, and lead may beemployed.

The ferrous particle content of the composition may be in the form ofpure iron, iron oxide (FeO), millscale Fe,0., a partially reduced ironfor example a 30% reduced iron (30% Fe O or mixture of any of these. If100% millscale (Fe O (86% iron) is used, there is considerable loss ofthis magnetite because of the reducing action during sintering. Themillscale can also be used in partially reduced condition. Examples ofcommercially available iron powders that may be used are Hysqvarna iron,an electrolytic iron made by Huskavarna of Sweden, I-Ioeganses Anchor 80iron, a reduced magnetite iron made by Hoeganses Sponge IronCorporation, of Riverton, New Jersey, and Pyron'iron, a reducedmillscale type iron made by Puron Iron Corporation, Niagara Falls, NewYork.

We can also use ferrous particles which are iron alloys. The extent ofalloying ingredients in the iron will preferably not exceed about 40% byweight of the particle. For example nickel-iron and manganese-iron alloyparticles may be used and will preferably contain in greatest amountabout 10% by weight of nickel'and/or manganese. When silicon or chromiumare the alloying ingredients these will preferably not exceed by weightof the particle. The latter two alloying constituents are especiallydesirable where corrosion resistance is to be obtained.

The iron powders should preferably be in major amount of a size between200 to 325 mesh. Anchor 80 iron is about 75% of this size and thebalance is between 80 to 150 mesh.

In carrying out the process of the present invention, according to apreferred procedure, the mixture of the selected finely comminuted orpowdered ingredients is thoroughly blended in a conventional manner toproduce a mass in which the ingredients are substantially uniformlydistributed, using a wetting agent such as kerosene when graphite ispresent. The blended mass is then placed in a mold and pressed to thedesired form of the article or product by suitable dies using abriquetting pressure above 20,000 p.s.i., preferably at least 40,000p.s.i., and up to about 60,000 p.s.i. Under such pressures the particlesof the composition, if properly selected, are compacted closely togetherforming briquettes of sharply defined shapes that are self-sustaining.It is found that the density of the briquettes falls off rapidly whenthe pressing pressure is below about 40,000 p.s.i. During pressing thereis a cold welding of the iron particles at points of contact. 7

The green briquettes are then sintered ina suitable furnace at asuitable temperature between 1900-2300 F. (above the melting point ofthe copper when present) preferably 2000 F. to 2050 F. where pure ironpowders are employed in a reducing atmosphere for a period between about30 to 60 minutes at temperature. During sintering, diffusion occursacross the previously cold welded interfaces causing the particles togrow together and bond into a cohesive mass. It also eifects flow of anycopper present to aid in bonding and more importantly, densification ofthe mass. 7

To provide for a reducing atmosphere, the chamber of the furnace may beprovided with suitable inlets through which dry hydrogen, cracked gas,or other reducing gases may be supplied and which inlets may be later beused for feeding an oxidizing atmosphere such as air, steam, or oxygen.Moreover, the furnace may be electrically heated or gas fired externallyin any desirable manner so as to maintain the required sinteringtemperature.

When the briquettes have ben sufficiently sintered, the briquettes whilestill ina reducing atmosphere are permitted to cool to the temperaturedescribed above in the range 10502150 F. where change from a reducing toan oxidizing atmosphere is to take place, for instance, about 1500 F.where the composition is primarily'com posed of substantially pure ironpowders.

The furnace chamber is now cleared of the reducing atmosphere, and thenair, steam, oxygen, or other oxidizing atmosphere is admitted theretoand passed therethrough. While subjected to the oxidizing effects ofsuch oxidizing atmosphere, the briquettes are substantially uniformlycooled down to a temperature below which nonuniform cooling outside thefurnace will not promote warpage from thermally induced stresses,usually a temperature between 300 F. to 500 F. and preferably ambienttemperature, before being removed from the furnace. By preference theparts will be thus cooled at a rate between 50 to 500 F. per hour,preferably between 100 F. to 250 F. per hour in the case of pure ironpowders, for best oxidizing results.

During treatment, the austenite and/or subsequently formed pearlite orcementite formed by sintering will be partially decarburized. Thisprevents the formation of excessive amounts of pearlite and promotesinternal oxidation forming one or more of the iron oxides, magnetite (Fe0 Wiistite (FeO) and hematite (Fe O to produce a product dimensionallystable when subsequently exposed to high temperature application. It hasbeen found advantageous in using some oxidizing agents to modify thecooling procedure described above to initially cool the sintered productin the protective atmosphere down below the upper oxidizing temperature,for instance, to 300 F. or ambient temperature, then to reheat theproduct in either a protective or oxidizing atmosphere to the aforesaidoxidizing temperature and then to cool again in an oxidizing atmospherein the manner previously described.

If the composition initially contains magnetite as its essential ironingredient, the sintering is preferably carried out in dry hydrogen forbest reduction results, during which a 50% to reduction occurs. Thesintered product is then preferably cooled down to about 1500 F. inhydrogen and then cooled to room temperature in an oxidizing atmosphereas described above. Use of magnetite as the major iron constituent ispreferably limited to applications where shrinkage is not an importantdesign factor.

The tensile strength of the resultant sintered products of our inventionwill be between 12,000 to 17,500 p.s.i. at ambient' temperaturedepending upon the briquetting pressure (35,000 to 60,000 p.s.i.) usedand will be somewhat lower where the products have received theoxidizing treatment in cooling. Their strength'at 1200 F. in

the as-sintered and preoxidized condition will be about 6500 p.s.i.

The various effects made possible by application of one or more of thefeatures of our invention may to some extent be appreciated fromconsideration of a composition containing by weight 86% iron, 4% copper,and 10% graphite.

Thus cooling down the sintered briquette of such a composition in theconventional manner in dry hydrogen to ambient temperature results in aparticle structure principally of large particle areas of pearlite andferrite, and small areas of copper, which pearlite and ferrite, aspreviously described, will in service be oxidized at high temperature,and effect a growth of the structure to cause dimensional changes of amagnitude apt to cause distortion and other undesired effects.

On the other hand, with suflicient graphite present, such graphite willform a considerable part of any wearing surface of the structure toprovide initial dry lubrication and control the wear rate of the wearingsurface.

When, as previously described, the article is cooled in a protectiveatmosphere down to the transfer temperature or cooled to ambienttemperature in a protective atmos phere and subsequently reheated. tothe transfer temperature and then in either case cooled in air oroxygen, there are substantial areas of iron oxides from the groupmagnetite, wiistite and hematite formed, or areas of such oxidesassociated with graphite. Moreover, any partially decarburized andspheroidized pearlite will be surrounded by such iron oxides and/ oriron oxides and graphite.

After a thousand hours of service at 1200 F., the portion of thecomposition adjacent the wear surface isprincipally composed of the ironoxides, of magnetite, wiistite and hematite, substantially no pearlite,and some ferrite, and there is a change to magnetite of the partiallydecarburized and spheroidized pearlite areas produced during cooling.Such additional oxidation improves the wearing qualities without causingthe dimensional instability which is inherent in oxidation of astructure where there has been no pre-oxidation treatment of thestructure as in the present invention. The graphite present duringservice maintains the character of the previously formed iron oxides andimparts resistance to galling wear by dry film lubricant section.

The following examples illustrate a number of preferred embodimentswithin the scope of our invention:

Example I A green composition was prepared as described above containingby weight 86 parts of Anchor 80 iron powder, 4 parts of C copper andparts of Asbury F.G. graph ite. The composition was pressed in a die tothe shape of a flat seal for the regenerator of a gas turbine engine,the approximate size being 20 in outside diameter by 18 in insidediameter by in thickness. The briquette thus formed was then sintered ata temperature of 2050 F. for thirty minutes in a reducing atmosphereemploying dry hydrogen gas. After sintering, the part was cooled down toambient temperature in the presence of dry hydrogen. A strong, readilyhandled seal having a tensile strength above 12,000 p.s.i. was obtained.During sintering there was a slight shrinkage in the part which offsetthe spring-out in size due to release of elastic strain upon ejection ofthe part from the die. These changes in dimension generally balancedeach other so that the die cavity could be made size for size to thefinished dimension of the part. The seal exhibited considerable wearresistance in service on a turbine engine at 1200 F. Its wear resistancewas comparable, however, to a solid graphite member but possessed greatstrength as compared to graphite, and 20 to 50 times the oxidationresistance of graphite. The seal was, however, subject to dimensionalchange due to substantial growth during service, this latter conditionalso causing interference with other parts of the engine and producingbuckling. Hence a part as here made has a limited application to serviceconditions below 900 F. or at higher temperature where brief use isintended as in the case of missile parts, for example, seals orbearings.

Example II A seal was prepared in accordance with Example I andsubsequently steam treated for fifteen minutes in superheated steam atabout 900 F. After treatment, during which some growth took place, thepart was machined to print dimension and subjected to service at 1200 F.The part exhibited good wear resistance, but the treatment whileproducing a better product than the part prepared in accordance withExample I was not found to possess adequate dimensional stability forlong time service at high temperature, and therefore has similarrestricted application.

Example III A green compact was prepared in accordance with Example I inthe form of a seal and sintered at 2050 F. for thirty minutes in areducing atmosphere employing dry hydrogen. The sintered compact wasthen cooled in a reducing atmosphere employing dry hydrogen down to atemperature of approximately 1500 F. Air was then admitted to thefurnace after the hydrogen was purged therefrom and the part cooled inair at the rate of 100 to 200 F. per hour down to a temperature belowwhich non-uniform cooling outside the furnace did not promote warpagefrom thermally induced stresses. In the present example, the part wascooled down to 300 F. During this treatment certain seal dimensions grewabout 0.003" per linear inch. The part was then machined or ground toexact size. The disturbed metal area caused by.

grinding or machining was etched from the wearing surface and graphitewas then restored to its wearing surface by mechanical impregnation(rubbing) to minimize breakin-wear. The part was then placed in serviceat 1200 F. The part exhibited comparable wear resistance to the seals ofExamples I and II and was substantially dimensionally stable, that is tosay, that during service in the order of 100 hours at l2=00 F. the sealreadily conformed itself to changes in the mating regenerator partscaused by uneven heating and no buckling was observed, such as wouldoccur due to excess growth. The part had a tensile strength of about15,700 p.s.i. at room temperature and 6220 p.s.i. at 1200 F. It will beunderstood that by a few tests, it is possible to accurately estimatethe extent of growth in the briquette upon oxidation treatment and suchgrowth anticipated in the die design so as to avoid subsequent trimmingor machining of the part to finished dimensions and subsequentimpregnation with graphite.

Example IV A green composition as called for in Example I was pressed ina mold using a die to form a regenerator center bearing of approximately2%" O.D., I.D., and 2" length. The hearing was sintered at 2050 F. in areducing atmosphere employing dry hydrogen for thirty minutes and thencooled completely in dry hydrogen. This part was suitable for use whereclose tolerances in the bearing dimensions was not required in service.

Example V A hearing was prepared as in Example IV, but followingsintering the part was cooled in dry hydrogen to approximately 1500 F.and then air cooled at the rate of about 100 to 200 F. per hour down toa temperature of about 300 F. During sintering, there was a slightshrinkage which olfset the spring of the compact in the die and therewas a growth of about 3% upon oxidizing the part. Since no allowance hadbeen made for this growth, the bearing following cooling was machined toaccurate dimensions and was thereafter capable of use in service at 1200without dimensional change and was possessive of excellent wearcharacteristics.

Example VI A green compact was prepared in accordance with Example I inthe form of a seal and sintered at 2050 F. for thirty minutes in areducing atmosphere employing dry hydrogen. The sintered product wasthen cooled to ambient temperature in dry hydrogen after which it wasagain reheated to about 1500 F. in air and then cooled down again in airat the rate of about 100 to 200 F. per hour to ambient temperature. Theseal was thereafter placed in service and exhibited after 1000 hourssimilar wear characteristics and oxidation resistance to the product ofExample III.

Although several preferred embodiments of the invention are disclosed,it will be understood that numerous changes may be made in the time,temperature and rate of cooling relationship and in the proportions andcharacter of ingredients without departing from the spirit and scope ofour invention as set forth above and in the appended claims. providesuggested cooling rates and ranges for a composition such as describedin Examples No. III and VI for difierent starting temperatures ofoxidation treatment. We do not, however, desire to be limited thereto.

For example, the following table will We claim:

1. In a process of producing sintered powdered ferrous products havingsubstantial dimensional stability under service conditions attemperatures up to about 1200 F. the steps comprising briquetting acomposition containing at least about 79% by weight of ferrous powderparticles into a predetermined shape capable of being handled, sinteringthe briquettes in a reducing atmosphere at a temperature sufficientlyhigh to cause the ferrous particles to bond together and produce arelatively dense iron particle structure, cooling the sintered productin a reducing atmosphere and during processing substantially uniformlycooling the sintered product in an oxidizing atmophere from atemperature between 1050 F. to 2150 F. at a rate not exceeding about 500F. per hour.

2. In a process of producing sintered powdered ferrous products havingsubstantial dimensional stability under service conditions attemperatures up to 1200" F., the steps comprising briquetting acomposition containing at least about 79% by weight of powder particlesselected from the group consisting of iron and iron alloys and mixturesthereof into a predetermined shape capable of being handled, sinteringthe briquette in a reducing atmosphere at a temperature sufiicientlyhigh to cause the ferrous particles to bond together and produce arelatively dense iron particle structure, cooling the sintered productin a reducing'atmosphere to a temperature at which the product will notbe subject, to catastrophic oxidation, and during processingsubstantially uniformly cooling the product in an oxidizing atmosphereat the rate of about 50 F. to 500 F. per hour from said last mentionedtemperature to a temperature at which the product will not be subject towarpage upon exposure to cooling of a non-uniform character.

3. In a process of producing sintered powdered ferrous products havingsubstantial dimensional stability under service conditions attemperatures up to 1200 F., the steps comprising briquetting acomposition essentially containing at least about 79% by weight offerrous powder particles into a predetermined shape capable of beinghandled, sintering the briquette in a reducing atmosphere at atemperature between 1900 F. and 2300 F. sufi'iciently high to cause theferrous particles to bond together and produce a relatively dense ironparticle structure, cooling the sintered product in a reducingatmosphere to a temperature below about 2150 F. at which the productwill not be subject to catastrophic oxidation and during processingcooling the sintered product substantially uniformly in an oxidizingatmosphere from a temperature corresponding substantially to said lastmentioned temperature to a temperature in the order of 500 F. belowwhich the product will not be subject to warpage upon exposure tocooling of'a non-uniform character and at a rate between about 50 to 500F. per hour.

4. In a process of producing sintered powdered ferrous products havingsubstantial dimensional stability under service conditions attemperatures up to 1200" F., the steps comprising briquetting acomposition containing by weight at least 79% ferrous powder particlesand up to about 15% graphite powder particles into a predetermined shapecapable of being handled, sintering the briquette in a reducingatmosphere at a temperature between 1900 F. to 2300 F. sufficiently highto cause the ferrous particles to bond together and produce a rela-'oxidizing atmosphere'at the'rate of between about 50 F.

to about 500 F. per hour from a temperature of about 1050" F. to atemperature below about 500 5. In the process of producing sinteredpowdered metal products having good wearing qualities the steps'comprising briquetting a particle composition comprising 6% to 15% byweight of graphite and a remainder. comprising at least 79% of ferrousmetal into a predetermined shape capable of being handled, sintering thebriquette in a reducing atmosphere at a temperature between 1900 F. and2300 F., sufficiently high to cause the ferrous particles to bondtogether and produce a relatively dense ferrous particle structure,cooling the sintered product in a reducing atmosphere to a temperatureat which the product will not be subject to catastrophic oxidation andduring processing cooling the sintered product in an oxidizingatmosphere from a temperature below said last mentioned temperature,

6. In a process of producing sintered powdered ferrous products havingsubstantial dimensional stability and good wearing properties underservice conditions at temperatures up to about 1200 F., the stepscomprising briquetting a particle composition comprising 6% to 15% ofgraphite, 1 /2% to 6% copper, and at least 79% iron into a predeterminedshape capable of being handled, sintering the briquette in a reducingatmosphere at a temperature between 1900 F. to 2300 F. sufficiently highto reduce the copper to a liquid phase and to cause the iron particlesto bond together and produce a relatively dense iron particle structure,cooling the sintered product in a reducing atmosphere and duringprocessing cooling the sintered'product in an oxidizing atmosphere froma temperature between 1050 F. to 2150 F. to a temperature below about500 F.

7. In the process of producing sintered powdered ferrous products havingsubstantial dimensional stability under service conditions attemperatures up to 1200 F., the steps comprising briquetting acomposition comprising ferrous particles into a predetermined shapecapable of being handled, said composition containing at least about 79%by weight of ferrous particles, sintering the briquette in a reducingatmosphere at a temperature between 1900 F. to 2300 F. and sufiicientlyhigh to cause the ferrous particles to bond together and produce arelatively dense iron particle structure, cooling the sintered productin a reducing atmosphere down to a temperature between 1050 F. to 2150F. and then further cooling the thus cooled product in an oxidizingatmosphere to a temperature below about 500 F. and at a rate notexceeding about 500 F. per hour.

8. In the process of producing sintered powdered ferrous products havinggood wear properties under service conditions at temperatures up to 1200F., the steps com prising briquetting a composition comprising ironpowder particles in amount by weight at least 79% of the composition,copper between 1 /2% to 6% and graphite between 6% -to 15% undersuitable pressure into a predetermined shape capable of being handled,sintering the briquette in a reducing atmosphere at a temperaturebetween 1900 F. to 2300 F. and sufliciently high to convert the copperto a liquid phase and to cause the ferrous particles to bond togetherand produce a relatively dense iron particle structure, cooling thesintered product in a reducing atmosphere down to a temperature below2150 F. at which the product will not be subject to catastrophicoxidation upon further cooling and then further cooling the thus cooledsintered product in an oxidizing atmosphere at the rate of 50 to 500 F.per hour to a temperature at which the product will not be subject towarpage upon exposure to cooling of a non-uniform character.

9. In a process of producing sintered powdered ferrous products havingsubstantial dimensional stability under service conditions attemperatures up to 1200 F., the steps comprising briquetting acomposition comprising by weight 79% to 92 /2% ferrous powder particles,1 /2% to 6% copper powder and 6% to 15% graphite powder into apredetermined shape capable of being handled, sintering the briquette ina reducing atmosphere at a temperature between 1900 F. and 2300 F.sufficiently high to reduce the copper to the liquid phase and to causethe ferrous particles to bond together and produce a relatively denseiron particle structure having dispersed graphite, cooling the sinteredproduct in a reducing atmosphere to a temperature below which theproduct will not be subject to catastrophic oxidation, reheating theproduct to a temperature which is between 1050 F. and 2150 F. and thensubstantially uniformly cooling the sintered product in an oxidizingatmosphere to a temperature below about 500 F. at which the product willnot be subject to warpage upon further cooling of a non-uniformcharacter.

10. A process as claimed in claim wherein graphite is present in aboutby weight of the composition.

11. In the process of producing sintered ferrous products having goodwearing quality, the steps comprising briquetting a compositioncomprising essentially iron powder and between about 10% to graphiteunder suitable pressure between 20,000 to 60,000 psi. into apredetermined shape capable of being handled, sintering the briquettefor 30 to 60 minutes in a reducing atmosphere at a temperature betweenabout 1900 F. to 2300" F. sufficiently high to cause the briquetted ironpowder to bond together and form a relatively dense iron particlestructure and cooling the sintered product in a reducing atmosphere downto a temperature which is at least below about 2150 F., and at which theproduct will not be subject to catastrophic oxidation upon furthercooling and subjecting said product thereafter to cooling in anoxidizing atmosphere from a temperature between 1050 F. to 2150 F. to atemperature below about 500 F.

12. As an article of manufacture, a sintered powdered metal product fora seal or bearing operable at elevated temperatures up to 1200 F. andhaving substantial dimensional stability when exposed to said elevatedtemperatures, comprising a compacted and sintered matrix of bondedferrous particles having an oxidized grain structure which ischaracterized by the appearance of substantial amounts of iron oxidefrom the group consisting of magnetite, wiistite and hematite andmixtures thereof between and around particles of ferrite and pearlitefrom the group consisting of partially decarburized pearlite andspheroidized pearlite and mixtures thereof.

13. As an article of manufacture a sintered powdered metal product for aseal or bearing operable at elevated temperatures up to 1200 F. andhaving substantial dimensional stability when exposed to said elevatedtemperatures comprising a compacted and sintered matrix of bondedferrous particles having an oxidized grain structure which ischaracterized by the appearance of substantial amounts of iron oxidefrom the group consisting of magnetite, wiistite, and hematite, andmixtures thereof between and around particles of ferrite and pearlitefrom the group consisting of partially decarburized pearlite andspheroidized pearlite and mixtures thereof, said iron oxide beingpresent in amount at least 10% to by weight of the oxidized structure.

14. As an article of manufacture a sintered powdered metal product for aseal or bearing operable at elevated temperatures up to 1200 F. andhaving substantial dimensional stability when exposed to said elevatedtemperatures comprising a compacted and sintered matrix of bondedferrous and graphite particles having an oxidized grain structurecontaining dispersed graphite particles and characterized by theappearance of substantial amounts of iron oxide from the groupconsisting of magnetite, wiistite, and hematite and mixtures thereofbetween and around particles of ferrite and pearlite from the groupconsisting of partially decarburized pearlite and spheroidized pearliteand mixtures thereof.

15. As an article of manufacture a sintered powdered metal product for aseal or bearing operable at elevated temperatures up to 1200 F. andhaving substantial dimensional stability when exposed to said elevatedtemperatures and having good wear properties comprising a compacted andsintered matrix of bonded ferrous, copper and graphite particles havingan oxidized grain structure containing dispersed graphite particles andcopper and characterized by the appearance of substantial amounts ofiron oxide from the group consisting of magnetite, wiistite, andhematite and mixtures thereof between and around particles of ferriteand pearlite from the group consisting of partially decarburizedpearlite and spheroidized pearlite and mixture thereof.

16. As an article of manufacture a sintered powdered metal product for aseal or bearing operable at elevated temperatures up to 1200 F. andhaving substantial dimensional stability when exposed to said elevatedtemperatures comprising a compacted and sintered matrix of bondedparticles comprising by weight 79% to 92 /2% ferrous powder, 1 /2% to 6%copper powder, and 6% to 15% graphite powder, said matrix having a grainstructure containing dispersed graphite particles and copper andcharacterized by the appearance of substantial amounts of iron oxidefrom the group consisting of magnetite, wiistite and hematite andmixtures thereof between and around particles of ferrite and pearlitefrom the group consisting of partially decarburized pearlite andspheroidized pearlite and mixtures thereof.

17. As an article of manufacture for a seal or bearing operable atelevated temperatures up to 1200 F., a sintered powdered metal producthaving substantial wear qualities and dimensional stability when exposedto said elevated temperatures comprising an oxidized compacted andsintered matrix of bonded particles comprising by weight about to 94%ferrous powder and 6% to 15% graphite powder.

18. As an article of manufacture for a seal or hearing operable atelevated temperatures up to 1200 F., a sintered powdered metal producthaving substantial dimensional stability and Wear qualities when exposedto said elevated temperatures comprising an oxidized compacted andsintered matrix of bonded particles comprising by weight about 86 partsof iron powder, about 4 parts of copper powder and about 10 parts ofgraphite powder.

19. As an article of manufacture for a seal or bearing operable atelevated temperatures up to 1200 F., a sintered powdered metal producthaving substantial dimensional stability and wear qualities when exposedto said elevated temperatures comprising an oxidized compacted andsintered matrix of bonded particles comprising by weight 79% to 92 /2%ferrous particles selected from the group consisting of iron and ironalloys containing up to 40% by weight of alloying ingredients, andmixtures of said iron and iron alloys and 6% to 15 graphite particles,said oxidized matrix having a grain structure characterized by dispersedgraphite and by the appearance of substantial amounts, at least 5% to20% by volume of the internally oxidized areas, of iron oxide from thegroup consisting of magnetite, wiistite and hematite and mixturesthereof between and around particles of ferrite and of pearlite from thegroup consisting of partially decarburized pearlite and spheroidizedpearlite and mixtures thereof.

20. In a process of making a seal between a pair of parallel surfaces ofparts having relative motion with respect to each other and at least oneof which is subject to a temperature up to about 1200 F., the stepscomprising briquetting a composition comprising ferrous powder particlesand graphite into a shape to produce said seal and capable of beinghandled, sintering the seal forming briquette in a reducing atmosphereat a temperature suificiently high to cause the ferrous particles tobond together and produce a relatively dense iron particle structure,cooling the sintered seal in an oxidizing atmosphere from a temperaturebetween 1050 F. to 2150 F. to a temperature below about 500 F. andthereafter assembling said seal between said parallel surfaces.

References Cited in the file of this patent UNITED STATES PATENTS LenelJan. 16, 1940 Thomson Oct. 14, 1958 Shaw et al. July 19, 1960 Grant etal. Dec. 25, 1962

1. IN A PROCESS OF PRODUCING SINTERED POWDERED FERROUS PRODUCTS HAVINGSUBSTANTIAL DIMENSIONAL STABILITY UNDER SERVICE CONDITIONS ATTEMPERATURES UP TO ABOUT 1200* F. THE STEPS COMPRISING BRIQUETTING ACOMPOSITION CONTAINING AT LEAST ABOUT 79% BY WEIGHT OF FERROUS POWDERPARTICLES INTO A PREDETERMINED SHAPE CAPABLE OF BEING HANDLED, SINTERINGTHE BRIQUETTES IN A REDUCING ATMOSPHERE AT A TEMPERATURE SUFFICIENTLYHIGH TO CAUSE THE FERROUS PARTICLES TO BOND TOGETHER AND PRODUCE ARELATIVELY DENSE IRON PARTICLE STRUCTURE, COOLING THE SINTERED PRODUCTIN A REDUCING ATMOSPHERE AND DURING PROCESSING SUBSTANTIALLY UNIFORMLYCOOLING THE SINTERED PRODUCT IN AN OXIDIZING ATMOSPHERE FROM ATEMPERATURE BETWEEN 1050*F. TO 2150*F. AT A RATE NOT EXCEEDING ABOUT500*F. PER HOUR.
 12. AS AN ARTICLE OF MANUFACTURE, A SINTERED POWDEREDMETAL PRODUCT FOR A SEAL OR BEARING OPERABLE AT ELEVATED TEMPERATURE UPTO 1200*F. AND HAVING SUBSTANTIAL DIMENSIONAL STABILITY WHEN EXPOSED TOSAID ELEVATED TEMPERATURES COMPRISING A COMPACTED AND SINTERED MATRIX OFBONDED FERROUS PARTICLES HAVING AN OXIDIZED GRAIN STRUCTURE WHICH ISCHARACTERIZED BY THE APPEARANCE OF SUBSTANTIAL AMOUNTS OF IRON OXIDEFROM THE GROUP CONSISTING OF MAGNETITE, WUSTITE AND HEMATITE ANDMIXTURES THEREOF BETWEEN AND AROUND THE PARTICLES OF FERRITE ANDPEARLITE FROM THE GROUP CONSISTING OF PARTIALLY DECARBURIZED PEARLITEAND SPHEROIDIZED PEARLITE AND MIXTURES THEREOF.